1. Field of the Invention
The invention relates to a protective switching apparatus or system comprising between the two connection terminals connected respectively to the electric mains and to a load, on the one hand, a first switch which is controlled by an electromagnet and which is adapted to establish and interrupt frequently the nominal current of the load, and on the other hand a second switch with automatic opening having properties for limiting fault currents such as those of short circuits, so as to ensure protection of the lines and prevent damage to the first switch.
2. Description of the Prior Art
In known systems or apparatus, which correspond to the general construction mentioned above, different types of limiting switches are met with having special properties, for which they are preferably chosen depending on the nature of the lines, of the load, of the ability of the first switch to withstand overload , and, also, on the probable frequency of the appearance of faults.
These limiting switches may be fuses, mechanical switches with high speed opening comparable to those of circuit breakers, switches with automatic opening which use repulsion forces, or switches with an isolating screen in which this latter provides rapid shearing of the arc.
If no other precautions are taken other than those consisting in placing the two switches in series, force compensation devices must be associated with the first switch for avoiding lifting of the mobile contact under the effect of repulsion forces which develop locally; these devices generally comprise magnetizable pieces whose behavior in the presence of large currents counterbalances the electrodynamic repulsion forces which cause such lifting.
These devices and precautions however do not prevent the first switch from having very high currents flowing therethrough and which consequently its geometry must take into account. It has also been proposed to provide the mobile contacts of the first switch with very powerful pressure springs for avoiding the above mentioned lifting, but this measure requires the use of an electromagnet which is so powerful that the contactor apparatus must then be totally reconstructed.
Finally, it has been proposed to downgrade the contactor apparatus used in the above mentioned circumstances, that is to say to use a first switch belonging to an apparatus suitable for a much higher nominal current; such a solution is obviously costly, and results further in a material over load of the installations.
The invention proposes consequently providing a combined apparatus such as the one whose general construction is defined above with robust means adapted for transferring a considerable fraction of the overload current into a parallel channel.